10         20         30         40         50         60
         |          |          |          |          |          |
MSYRKELEKY RDLDEDKILG ALTEEELRKL ENELEELDPD NALLPAGLRQ RDQTQKPPTG - 60
PFKREELMAH LEQQAKDIKD REDLVPFTGE KRGKAWIPKQ KPMDPVLESV TLEPELEEAL - 120
ANASDAELCD IAAILGMHTL MSNQQYYEAL GSSTIVNKEG LNSVIKPTKY KPVPDEEPNS - 180
TDVEETLKRI QNNDPDLEEV NLNNIMNIPV PTLKALAEAL KTNTYVKKFS IVGTRSNDPV - 240
AFALAEMLKV NNTLKSLNVE SNFISGSGIL ALVEALQSNT SLIELRIDNQ SQPLGNNVEM - 300
EIANMLEKNT TLLKFGYHFT QQGPRLRASN AMMNNNDLVR KRRLAELNGP IFPKCRTGV

Tropomodulin is a 40-kDa protein that stabilizes the actin-tropomyosin filament by capping the slow-growing end (P-end). The molecule was originally found as a tropomyosin-binding protein within the spectrin network that lines the erythrocyte membrane and was later also identified as the P-end capping protein of muscle thin filaments. Now four isoforms have been identified in a variety of mammalian cell types. Tropomodulin inhibits the polymerization and depolymerization of actin monomers at the P-end. This capping is not static and fixed, but dynamic, allowing an exchange of actin monomers with a definite lifetime. Tropomodulin could play important roles in defining actin filament lengths especially in the skeletal muscle sarcomere. This hypothesis is supported by the recent finding that tropomodulin interacts with the N-terminal segments of nebulin, which could be a molecular ruler, but it remains to be proved. Tropomodulin is a bifunctional molecule; the N-terminal half accommodates the interacting site for the N-terminal region of tropomyosin, whereas the C-terminal half is essential for its capping activity, presumably because it contains the actin-binding site. The nebulin interaction site has not been mapped on tropomodulin.